1. Field of the Invention
The present invention relates to an optical filter and an optical device. Priority is claimed on Japanese Patent Application No. 2003-354027, filed Oct. 14, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
A fluorescent microscope is an example of an optical device that is used in the observation of living biological specimens. The fluorescent microscope is a device that irradiates with excitation light a specimen such as cells that have undergone a staining treatment, and observes the fluorescent light emitted by the specimen at that time. By carrying out observation using this type of device, it is possible to analyze the structure and properties of the specimen.
In the recent analysis of the genome, for example, it is necessary to irradiate excitation light having a wavelength of 502 nm and observe fluorescent light having a peak at 526 nm. In this case, because the wavelengths of the excitation light and the fluorescent light are close to each other, it is necessary to reliably separate both wavelengths and detect the fluorescent light with a high efficiency. In order to attain this, it is necessary to eliminate the excitation light in the rejection band and transmit the fluorescent light having the wavelength to be observed in the transmission bands. An optical filter having such properties is used as a crucial part that determines the sensitivity and precision of the fluorescent light measurement.
In this optical filter, there is a steep rise in the spectral characteristics at the boundary between the transmission bands and the rejection band, and in the transmission bands, a capacity to transmit substantially 100% of the light is required.
Such an optical filter that blocks light of a predetermined wavelength band and transmits the other wavelengths is called a minus filter. This optical filter (minus filter) is manufactured as a multi-layer film, in which a layer having a high refractive index and a layer having a low refractive index are alternately stacked on a substrate.
In an optical filter whose refractive indices of the films in the direction of the film thickness are periodically and continuously varied is called a rugate filter. Here, when the refractive index distribution is in a form called a wavelet, it is possible in principle to eliminate periodic variation in the transmissivity (ripple) in the transmission bands (for example, W. H. Southwell, “Using Apodization Function to Reduce Sidelobes in Rugate Filters”, Appl. Opt., 1989, Vol. 28). In addition, accompanying this, as the number of layers described above is increased, the rise at the boundary between the transmission bands and the rejection band can be made more steep. The use of this principle is shown in FIG. 14A.
However, it is extremely difficult to vary continuously the refractive indices of the films during actual film formation. Thus, various proposals have been made in which the continuous refractive index distribution is partitioned stepwise and varied by approximation (for example, Japanese Patent No. 3290629, FIG. 1). The use of this principle is shown in FIG. 15A.